To activate tag mode, you merely talk to the Inn's proprietor, select "tag mode," and walk around with your Nintendo DS on your person. The DS can (and in fact, should) be in sleep mode. If you pass in the vicinity of another player whose Nintendo DS and copy of Dragon Quest IX is also in tag mode, his or her character will "visit" the Quester's Rest and hang out. He or she might also provide you with a coveted treasure map.
The only problem with Dragon Quest IX's tag mode is that you need at least one other fan of the game to make it work. And what are the chances that you're going to run into another adventurer in tag mode just by walking around the shopping mall?
I decided I'd put the popularity of Dragon Quest IX to the test at Otakon 2010. I carried my Nintendo DS and Dragon Quest IX in tag mode during the two busiest days of the convention (Friday and Saturday) to see what I could see.
I was, to put things lightly, floored by the results. I attracted 53 guests in total. I undoubtedly could have garnered more, but once you tag three people, the game stops canvassing for guests until you meet your three latest additions and go out canvassing again. Otherwise, I might have topped 99, as it never took more than ten minutes to fill up the three parking spots.
As you collect more and more guests, your Inn becomes bigger and fancier. In time, you'll start finding valuable treasures and alchemy ingredients in guest rooms. If you're going to a game-related gathering or convention, don't leave home without Dragon Quest IX. The minimal work that goes into canvassing Inn guests is well worth the reward.